In the diagnosis of certain illnesses, radioactive agents are administered to patients. These administered agents have the characteristic of localizing in certain tissues and either not localizing, or localizing to a lesser degree, in other tissues. For example, iodine 131 will localize in thyroid glands. A representation of the spatial distribution and concentration of administered iodine 131 in a thyroid gland provides an image of the gland itself which is useful in diagnosing the condition of the gland.
In many studies, devices known as cameras which remain stationary with respect to the patient produce a representation of the spatial distribution of radioactivity. With commercially successful cameras, known as Anger-type cameras, a relatively large disc-shaped scintillation crystal is positioned to be stimulated by radiation emitted from the patient. A collimator is interposed between the patient and the crystal so that, for example, with a parallel hole collimator the rays striking the crystal are all generally perpendicular to it.
When incident radiation collides with a scintillation detector or crystal, the collision excites an electron into a higher energy state. When the electron drops back into its ground state, photons are emitted within a visible spectrum and can be observed by appropriately positioned photodetectors.
The conversion of energy from the radiation into light energy can occur in different ways. In a so-called photo-electric recoil occurrence, the entire energy of the gamma ray is used to excite a bound electron which then decays providing a photon. When this occurs, the gamma radiation is totally dissipated into the creation of the photon.
A second possibility is that the gamma radiation can produce visible light radiation by means of a Compton event. The gamma radiation interacts with a free electron, but rather than giving up all its energy to the electron which later is converted to light, only a portion of the energy is transmitted to the electron. This results in a scattering of the gamma radiation and produces visible light at the same time. The scattered gamma radiation can move a further distance through the scintillation crystal and later cause other photon-producing events including a photo-electric recoil of the remaining energy. With an Anger-type camera, photopeak events which are either both single photo-electric recoils and sequential events are used to produce images.
A third event that can produce visible light within the scintillation crystal is energy conversion of the gamma radiation into a positron-electron pair. The positron quickly recombines with an additional free electron in the crystal in a positron-electron annihilation. When the positron and electron annihilate each other, two gamma rays of energy 511 keV each are produced which then interact in one of the previously described ways. Typically, positron radiation is not used with Anger-type cameras.
When the crystal of an Anger-type camera scintillates, light is conducted through a suitable light pipe, to an array of phototubes. When a phototube is stimulated by gamma-generated light from a crystal, an electrical signal is emitted which is proportional to the intensity of light energy received by that tube. When a scintillation causes all or substantially all of the phototubes to emit signals, these signals are emitted concurrently and are then summed to provide a signal known as the Z signal used for energy resolution. For energy resolution, this Z signal is conducted to a pulse-height analyzer to determine whether the energy of the signal reflects the occurrence of a photopeak event caused by the isotope which has been administered to the patient. That is, the Z signal is of appropriate strength to reflect the full conversion of the energy of a gamma ray emitted from the administered isotope to light energy by the crystal.
Summing and ratio circuits are also provided which develop coordinate signals known as X and Y signals. These X and Y signals cause a dot to be produced on the screen of the oscilloscope at a location spatially corresponding to the location of the detected scintillation. Thus, the oscilloscope dots are displaced relatively, each at a location corresponding to the location of the corresponding scintillation in the crystal. The oscilloscope dots are integrated to produce an image.
The phototubes, the circuits and the oscilloscope function as a unit to provide a light amplifier such that each dot produced on the oscilloscope is a brightened representation of a scintillation. Through the use of persistence screen on the scope, or a photographic camera, dots produced in a study have been integrated to produce images.
With Anger-type cameras and other proposals which would, like the typical Anger camera, produce a plan view of a region under investigation, a photoelectric recoil has been considered to be an ideal event. That is, an event in which the entire energy of an incident gamma ray is converted to a light or an electrical output signal in a single interaction with a crystal or other transducer. A photoelectric recoil is desired because there is a single locus for spatial resolution. Since many of the output signals of a camera result from photopeak events which are in fact produced by a series of interactions, there is an inherent systemic limitation on the spatial resolution capabilities of a camera. The limitation results from the fact that loci of the interactions are spaced and resulting coordinate signals provide a "composite" locus offset from the true incident gamma ray path.
In order to minimize the inherent spatial error in an Anger-type camera, manufacturers have resorted to the use of very thin crystals. While the use of thin crystals enhances the spatial resolution of a camera, it reduces the efficiency of the camera because rays more readily pass through the crystal without full energy conversion to produce a photopeak event. It also precludes the use of certain higher energy isotopes such as 511 keV positron radiation because too low a percentage of the high energy rays result in photopeak events. In addition, the thin crystals are relatively weak and excessively susceptible to breakage which destroys their imaging usefulness.
A common systemic limitation of the capabilities of both Anger cameras and other prior proposals for gamma cameras is that each set of coordinate signals for spatial resolution and the associated Z signal for energy resolution derive from the same event. Since one event is used for both spatial and energy resolution, there is a consequent sacrifice of one or both of spatial resolution accuracy and camera efficiency.
With an Anger-type camera, the need for a collimator in all types of studies further limits the camera efficiency. A parallel hole collimator only passes of the order of ten percent of the patient-emitted energy to the crystal while a pinhole-type collimator is even less efficient.